The invention relates to a method for leading a web between calender roll nips. The invention also relates to a calender containing calender rolls that mutually form calender roll nips.
In papermaking, a normal finishing procedure for the paper web is calendering, in which the paper web is passed through several successive calender roll nips. The calendering is conducted either as on-line calendering in the same papermaking line in which the paper has been produced from pulp suspension and dried, or the calendering can be conducted as a separate processing stage for a previously produced and reeled paper web.
In the calendering, the thickness of the paper web as well as the quality of the surface is affected by means of a linear load effective in the nip between the calender rolls. Furthermore, the calendering result is affected by the temperature and the moisture content of the web. By means of the linear load, the temperature and the moisture content of the paper web it is thus possible to reach a desired result by setting the aforementioned variables to a suitable level.
The calendering, whether it takes place as an on-line calendering in a paper machine or as an off-line process, is typically conducted in a multiroll calender, in which the calender roll nips are formed between superimposed calender rolls forming a stack. The web to be calendered travels along a winding path via these nips in such a manner that it is brought to the upper end of the stack of calender rolls, and it leaves the calender in the lower end of the stack of calender rolls. To stabilize the travel of the web and to cool down the web, so called take-out leading rolls are used on both sides of the stack of calender rolls, by means of which the web can be drawn straight to the side after the nip and guided to the following nip by means of the roll. Even though it is possible to better control the travel of the web in the calender by means of the take-out leading rolls, the problem is that the web dries when it travels in loops apart from the calender rolls on both sides of the stack of calender rolls. Especially in on-line applications, higher surface temperatures of the calender roll are required, which causes an overdrying problem in the paper. Because of this, it is often necessary to wet the paper again between the calender rolls, even if the paper were left in a suitably wet state in view of the calendering process in the drying section preceding the calendering, or if the paper were wetted again to the inlet moisture of calendering.
Inside the web loop guided by the take-out leading rolls, an air pocket is produced which can cause additional problems. When the web travels in the loop it evaporates moisture and heat at constant speed from its outer surface. On the inner surface of the web, i.e. on the side of the air pocket, the web, however, is stabilized to a thermodynamic balanced state with the air of the air pocket, i.e. no heat or moisture is transferred to the air pocket. This situation, however, only exists in the middle of the web. In the air pocket on the edge area of the web there prevails a descending gradient of both moisture content and temperature in the direction of the outer edges of the web, because of a lower temperature and air humidity prevailing outside. This uneven cross profile of the humidity and temperature in the pocket immediately causes a mass and heat flow in the edge area of the web from the web to the pocket through diffusion and conduction, respectively. These flows are thus directed away from the web, i.e. the air pocket dries and cools only the edge areas of the web, whereas on the opposite side of the web, i.e. on the side of the outer curve of the loop, the drying and cooling is considerably more uniform. This causes significant problems in the control of the cross profile of the web, and it can be assumed that the air pockets have a considerable significance in the profile defects in the edge of the web detected in different measurements.
The calender equipped with take-out leading rolls also contain temperature deviations caused by other factors. For example the heat generation of the take-out leading rolls themselves at the location of the bearings located inwardly from the ends of the rolls, cause clear heat peaks in the otherwise even middle area.
Furthermore, present multiroll calenders evaporate water efficiently, because the aim is to use high surface temperatures of the rolls. The wetting requirement is typically 0.5-1.5 g/m2 of paper. In addition to profile defects, the problem in multiroll calenders is the wetting efficiency by present wetting methods. The heating up of the rolls also involves a poor coefficient of efficiency.
U.S. Pat. No. 4,642,164 discloses a manner for affecting the conditions prevailing inside the air pocket by placing a steam box therein from which it is possible to supply desired amounts of steam in the lateral direction of the web in zones on the inner surface of the web. Despite of the steam supply, the drying of the edges poses a problem, and in order to attain the best result, it should be possible to supply steam very accurately in the outer edge zones. The efficiency in this wetting inside the air pocket suffers from the fact that heat and moisture escape outside the pocket.
Furthermore, Finnish patent 92850 discloses an encapsulated supercalender, wherein the moisture level can be increased around the entire calender.
It is an aim of the invention to eliminate the aforementioned drawbacks and to introduce a method by means of which the profile defects can be better avoided and wetting and/or heating can be enhanced in a relatively simple manner. Another aim is to be able to better control the calendering process by means of procedures conducted inside the web loops that run outside the stack of calender rolls. In the air space inside the web loop, the flows are restricted in the transverse direction of the web. An efficient restriction is attained by closing both open ends of the pocket formed by the web loop. In a substantially closed space it is possible to much more efficiently affect the web passing by as well as the roll located by the space, and to prevent the development of gradients in the edge areas of the web as well as to prevent the drying of the edge areas.
It is also an aim of the invention to present a device for guiding a web between calender roll nips, by means of which it is possible to avoid the drawbacks due to the previous structures. To attain this purpose, the device is primarily characterized in what will be presented in the characterizing part of the appended claim 11. The device comprises obstacles placed in connection with the web loops outside the stack of calender rolls, which obstacles are arranged to prevent the flow of air in the air pocket in the transverse direction of the web. The obstacles can be walls closing the air pockets entirely or partly at the ends, and/or partitioning walls closing the pocket likewise either partly or entirely in its cross direction. As for the other preferred embodiments of the device according to the invention, reference is made to the appended dependent device claims.